The California Department of Water Resources suspended overflow from the Oroville Dam after gushing water ripped open a sinkhole in the middle of the spillway. Engineers assessed the options and opened up a secondary, emergency spillway on Saturday morning, February 11.

Photograph by Randy Pench/The Sacramento Bee via AP

Huge Sinkhole Opens at Highest Dam in the U.S.

As heavy rains fill Lake Oroville in California, the overflow damages a spillway and forces an emergency diversion of water.

Update: Though the California Department of Water Resources previously insisted there was no danger to the surrounding towns, 188,000 people have now been evacuated from the nearby area as water levels continued to rise and overflow the spillway. The situation stabilized late Sunday night, February 12, but roads were clogged as residents continued to attempt to leave.

After a year of heavy rains, California’s Lake Oroville—home to the country’s tallest dam—has almost reached its maximum capacity, and its infrastructure is feeling the strain.

That’s a complete reversal from the end of 2015, when persistent drought brought the lake’s water level to about 694 feet, or 33% of capacity—the second lowest level ever recorded.

By yesterday morning the lake was swollen with so much water, 895 feet, that it had reached 96% of its maximum capacity.

The 770-foot-tall Oroville Dam creates the lake by impounding the Feather River, serving to generate hydroelectric power, supply water, and control flooding.

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Looking down a concrete ramp at Bidwell Marina on Lake Oroville in 2014, this photo shows the water level at an almost record low, with the sides of the lake basin left high and dry. Photograph courtesy the California Department of Water Resources

Unexpectedly heavy rains and rapidly-rising water levels have contributed to some infrastructure challenges that now have engineers scrambling to keep up with the water.

An emergency spillway, which has not been used since the dam’s completion in 1968, was opened Saturday morning after previous efforts to contain the water did not prove sufficient.

The California Department of Water Resources upped the output of water from 40,000 to 65,000 cubic feet per second in an attempt to avoid the use of the emergency spillway. But the torrent of water damaged the main spillway, opening a sinkhole in the middle of the passage. The cavity grew so big that it seemed to swallow the workers who climbed in to inspect the damage.

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The California Department of Fish and Wildlife moved millions of baby salmon from the Feather River Fish Hatchery in Oroville, California, after damage to Oroville Dam's spillway polluted the water at the hatchery. Photograph courtesy the California Department of Water Resources

Engineers were forced to temporarily stop the water flow after the extent of the damage became apparent.

This incident has had an impact on surrounding waterways. Four million baby salmon were recently moved from the nearby Feather River Hatchery because mud from the sinkhole has seeped into their water, putting their lives in danger.

According to the Water Department, Oroville Dam itself is sound–it is the spillway that threatens to flood the surrounding area.